picom doesn't support window managers that use containers

[gijsbers]

There's a bug in picom which becomes apparent when using plank and picom under window manager that use containers, like icewm, fluxbox and blackbox. At the bottom of the screen there is an area where picom interferes with the normal use of the screen. It changes the mouse and prevents normal interaction with the root window. This doesn't happen without picom. It only happens with WMs that use containers. Reparenting WMs add a frame window around a client application window. Some WMs have an extra level of nesting. They add a container to increase the isolation between the WM and application window. This is a technique that has been used since at least the turn of the century, but which seems not to be supported by picom. Issue #1081 could also be caused by this. Maybe #689 as well?